There are numerous varieties of documents that imply, by mere possession, that the holder is empowered to perform certain actions, or is the recipient of certain rights. Bearer bonds, for example, are presumed to be the property of the holder, who may sell or transfer the bonds to another. A driver's license carries with it the presumption that the holder is entitled to operate a motor vehicle.
Of course, some method of authentication is normally associated with each of these documents. Driver's licenses are normally printed on unique forms with a photo of the licensee incorporated into the form, while bearer bonds are printed on special paper with an embossed seal and a certificate number. But a counterfeiter who can obtain the proper form stock can still create duplicate certificates that are difficult to distinguish from legitimate documents.
It has long been known that certain materials possess inherent random characteristics that are useful for document authentication. Paper, for example, has a randomly varying opacity characteristic that can be detected by a transmissivity device. In such a device, a narrow, high-intensity light is directed through the paper and detected by a sensitive photodetector positioned on the opposite side of the paper from the light source. If this varying opacity characteristic is read along a specific portion or portions of the paper, and the resulting opacity values are recorded somewhere on the document (and/or in a separate data base), the opacity test process can be repeated and compared to the stored values to authenticate the document.
It is also known that magnetic materials exhibit an inherent random characteristic that is suitable for authentication purposes. In general, when a magnetic medium is deposited on a carrier material, such as paper or plastic stock, and the medium is read via a sensitive magnetic read head, a noise-like characteristic can be discerned. This characteristic has been demonstrated to be repeatable, and can be useful for authentication under proper conditions.
A method for enhancing this randomly varying characteristic of a magnetic medium is set forth in U.S. Pat. No. 5,177,344, the disclosure of which is fully incorporated by this reference. In this '344 patent, a process is described in which random signals are generated and used to control the character and duration of zones of altered magnetic remanence in a slurry of magnetic material. The slurry is then hardened so that the alterations introduced by the process cannot be changed or easily duplicated by a would-be counterfeiter.
Although these types of random variations have proven to be useful in authentication, there are applications in which mathematically deterministic random variations in magnetic characteristics are useful. For example, with a deterministic random variation, a reading device need look for only a certain kind of randomly appearing characteristic upon which to base its judgment of authenticity. Thus, there is a need for the capability to combine random and deterministic effects to yield more flexible security characteristics.
Accordingly, a need arises for a method and apparatus that permanently alters a magnetic characteristic of a medium in response to predetermined parameters. The process should be flexible enough to permit deterministic random variations of magnetic characteristics to create a magnetic characteristic of a medium that cannot be changed or easily duplicated after its creation.